In a daisy wheel printer, it is necessary, in accordance with character selection by keyboard, computer or other character selecting means, to move the selected character to an operative position for imprinting of said character by an impacting mechanism such as a hammer. For this purpose, the position of the daisy wheel as it rotates must be precisely monitored. Commonly, a position transducer in the form of an encoder is mounted on the daisy wheel shaft or on the output shaft of a motor, i.e. a servomotor or stepper motor, which is used to rotate the daisy wheel to bring the selected character to the operative position. An alternative and inexpensive means for providing positional information is to observe the stalks of the daisy wheel as it rotates with a light beam or other non-contacting means which is interrupted by the moving stalks. The observing sensor produces pulses representing quantized positional information as to the rotational position of the daisy wheel (relative to a datum or home position). However, because each stalk passing the sensor only produces one pulse, the resolution of the positional information is poor, and there is a degree of uncertainty as to the precise position of the daisy wheel.
It is an object of this invention to enable positional information of increased resolution to be obtained with a non-contacting sensor.